The conversion of diesel engines into natural gas operation has been an aspiration of the internal combustion engine industry for a period of time. Natural gas is a clean burning fuel (relative to diesel) with improved emission levels of both nitrogen oxides (NO.sub.x) and particulate matter. A known technique in the art for converting diesel engines to natural gas operation is called dual-fuel operation. Typically, in this method, natural gas is mixed with the intake air prior to the introduction of the air/natural gas mixture into the engine cylinder (a process known in the art as fumigation). The homogeneous mixture is then introduced into the piston cylinder during the intake stroke. During the compression stroke, the pressure and temperature of the homogeneous mixture are increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel is used to ignite the air/natural gas mixture. The advantage of the homogeneous mixture of air and natural gas is that the combustion fuel to air ratio (F/A) can be controlled, so as to cause the fuel to burn in a "propagation combustion mode". In this propagation combustion mode, it is possible to realize the advantages of "lean burn" operation, including lower NO.sub.x emissions, lower particulate matter, and efficient combustion. This dual fuel method has two main disadvantages, however. The first main disadvantage is encountered at high load engine operating conditions, when the elevated temperature and pressure in the piston cylinder during the compression stroke makes the air/natural gas mixture susceptible to premature detonation, or "knocking". Knocking is an uncontrolled combustion process and it can damage engines. Measures to reduce the risk of knocking include lowering the compression ratio of the piston stroke or limiting the power and torque output, but these measures cause a corresponding reduction in the engine's cycle efficiency (that is, not as much power is available from each piston stroke). The second main disadvantage is that under low load engine operating conditions, the mixture of fuel and air becomes too lean to burn. The intake air flow can be throttled to maintain flammability of the mixture, but this adversely affects the engine efficiency.
Recently, a different type of dual fuel combustion engine, herein referred to as a "high pressure direct injection" (HPDI) gas engine has become known in the art. Similar to the conventional dual fuel process described above, HPDI gas engines burn a large quantity of gaseous fuel, yielding an improvement (over diesel engines) with respect to the emission levels of NO.sub.x and particulate matter. In addition, HPDI gas engines purport to achieve the same combustion efficiency, power and torque output as state of the art diesel engines. The operational principle underlying HPDI gas engines is that two fuels are injected under pressure into the chamber near the end of the compression stroke. According to one method, a small quantity of "pilot fuel" (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous fuel. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the gaseous fuel that might otherwise be difficult to ignite. Known HPDI gas engines have no pre-mixture of fuel and air; as a result, they operate in a "diffusion combustion" mode, rather than propagation combustion mode. In a diffusion combustion mode, the bulk of the combustion is believed to occur in a local near-stochiometric reaction zone, where the temperature and resulting NO.sub.x formation are relatively high (compared to the temperature and resulting NO.sub.x formation caused by a lean burn propagation combustion mode).
In U.S. Pat. No. 5,365,902 (hereinafter referred to as the '902 patent), a method and apparatus for dual fuel injection is disclosed, which combines some of the advantages of diffusion combustion and propagation combustion. According to the '902 patent, the engine load conditions are detected, and under low load conditions, the pilot fuel is injected into the cylinder prior to the injection of the main gaseous fuel. When the main fuel is injected after pilot fuel injection, it combusts in a diffusion mode shortly after entering the combustion chamber. Alternatively, under high load conditions, the main gaseous fuel is injected into the combustion chamber prior to the injection of the pilot fuel. In this manner, the main gaseous fuel that is injected prior to the introduction of the pilot fuel will mix with air in the combustion chamber, so as to form a homogeneous mixture and burn in a propagation type combustion mode.
The principal drawback of the dual fuel injection technique disclosed in the '902 patent is the risk of premature ignition (knocking) under high load conditions. As the load is increased, the required fuel to air (F/A) ratio of the early-injected main gaseous fuel is increased. When the F/A ratio is high, there is a risk that compression (and the resulting increase in temperature and pressure) of the early-injected main gaseous fuel will cause it to ignite prior to the injection of the pilot fuel. This limit on the quantity of early-injection main gaseous fuel is referred to in this application as the "knock limit". Because the device disclosed in the '902 patent is knock limited, it can not be used for engines which target a high power density, nor can it operate efficiently under high load conditions. As mentioned above, knocking can also cause damage to the engine, reduce engine durability, limit the range of gaseous fuel quality or fuel composition that can be used, and limit the engine's power output.
U.S. Pat. No. 5,329,908 (hereinafter referred to as the '908 patent), discloses a method of using high pressure direct injection of gaseous fuel. According to the method taught by the '908 patent, gaseous fuel is injected at high pressure, near top dead center of the piston's compression stroke, thereby providing diesel-like combustion efficiencies. When the pressure of the gas reservoir drops below about 2,000 pounds per square inch, a controller changes the injection mode, causing the gaseous fuel to be introduced to the cylinder much earlier. For example, the gaseous fuel may be injected during the conventional intake stroke (that is, when air is being drawn into the combustion chamber). In this manner, the gaseous fuel is caused to mix with the air in the combustion chamber, forming a homogeneous mixture that will burn in a propagation mode of combustion. The method disclosed in the '908 patent requires a spark or glow plug to ignite the gaseous fuel in the combustion chamber.
The method disclosed in the '908 patent has several disadvantages. When operating in its early injection mode (that is, below 2000 pounds per square inch), it is subject to knocking, as discussed above, limiting engine efficiency and power density. Also, under low load conditions, the engine will reach a limit where it can not burn the early-injected gaseous fuel efficiently, because the (F/A) ratio of the mixture is too low. This low load limit, resulting from a low F/A ratio, is referred to in this application as the "flammability limit". The method disclosed in the '908 patent also relies on a glow plug or spark plug, yielding different combustion characteristics than fuel ignited by pilot diesel. In general, conventional systems that employ glow plugs or spark plugs require a significant quantity of flammable mixture to be formed prior to ignition, which results in large heat release rates and relatively high NO.sub.x formation.
PCT/International Publication No. WO 96/03578 (hereinafter referred to as the '578 PCT publication) discloses a method of reducing NO.sub.x and particulate matter emissions in diesel internal combustion engines. The '578 PCT publication discloses a technique of pre-injecting a small quantity of diesel fuel into the combustion chamber during the intake stroke, so that during the compression stroke, partially oxidized and/or peroxidized products with strong chemical affinity are formed. These products act as combustion accelerators for the main injection phase, reducing NO.sub.x and particulate matter emissions and noise produced during combustion. One drawback of the method disclosed in the '578 PCT publication is that the entire quantity of diesel fuel injected during the intake stroke does not contribute to the power stroke. As a result, there is a consequential reduction in efficiency. The '578 PCT publication discloses a single fuel method that employs a diesel pre-injection phase PCT publication does not disclose a dual fuel method or the use of a gaseous fuel source.
U.S. Pat. No. 5,711,270 (hereinafter referred to as the '270 patent) discloses a technique for high pressure injection of both oil and gaseous based fuel. The timing and the quantity of gaseous fuel injection is varied when the engine is under different load conditions, and the ignition of the fuel is always commenced with the introduction of the oil based fuel (that is, the pilot fuel) into the combustion chamber. The '270 patent discloses one method of implementing the aforementioned HPDI process. Because the '270 patent discloses always injecting the pilot fuel first, to initiate combustion, followed by the injection of the gaseous fuel, this method shares the same major disadvantages as other known HPDI methods, which include constantly burning in a diffusion mode of combustion and not being able to take advantage of lean burn combustion in a propagation mode, which yields lower NO.sub.x and particulate matter emissions.
The present dual fuel injection technique provides an improved method for the injection of fuel into the combustion chamber of an internal combustion engine. Preferably, the fuel consists primarily of a gaseous fuel, but also includes a more readily ignitable pilot fuel to assure ignition and to improve combustion characteristics.
The present dual fuel injection technique also provides a method of dual fuel injection into the combustion chamber of an internal combustion engine, which combines the advantages of lean-burn homogeneous propagation combustion over a range of operational conditions with some of the advantages of diffusion combustion in low and high load operational conditions.
The present dual fuel injection technique further provides a method of dual fuel injection into the combustion chamber of an internal combustion engine, which utilizes pre-mixed fuel giving rise to a lean burn propagation combustion under intermediate loads, but which does not suffer from a flammability limitation under low load conditions and does not suffer from a knock limitation under high load conditions.
The present dual fuel injection technique provides a method of dual fuel injection into the combustion chamber of an internal combustion engine, which retains the full high efficiency and high cycle output of high pressure direct injection (HPDI), retains the advantage of lower NO.sub.x and particulate matter emissions normally associated with lean burn propagation combustion of pre-mixed fuel, and achieves maximum power density in a knock resistant fashion.